1. Field of the Invention
This invention relates to an automatic equalizer for automatically equalizing a signal distorted by intersymbol interference.
2. Description of the Related Art
Conventional automatic equalizers include decision feedback type automatic equalizers and maximum likelihood sequence estimation type automatic equalizers.
A conventional decision feedback type automatic equalizer disclosed in the Journal of Papers of the Electronic Information and Communication Society of Japan B-II, Vol. J76-B-II, No. 4, p.191 is described with reference to FIG. 1.
Subtractor 401 subtracts estimated distortion signal Sed from received signal Sr and outputs equalized signal Seq which is improved in that the influence of distortion is reduced. Decision unit 402 receives equalized signal Seq as an input thereto, determines a value for distortion components to be removed from the received signal, and outputs detected signal Sd. Tap coefficient generation circuit 403 receives received signal Sr as an input thereto, produces an impulse response and outputs the impulse response as tap coefficient Tc. Transversal filter 404 receives detected signal Sd and tap coefficient Tc as inputs thereto to estimate distortion components and outputs estimated distortion signal Sed. Automatic equalization of the signal which has been distorted by intersymbol interference is performed by the processing described above.
Next, a conventional maximum likelihood sequence estimation type automatic equalizer disclosed in the Journal of Papers of the Electronic Information and Communication Society of Japan B-II, Vol. J76-B-II, No. 4, p.191 is described with reference to FIG. 2.
Subtractor 501 subtracts estimated received signal Ser from received signal Sr and outputs estimated error signal Seerr. Tap coefficient generation circuit 502 receives received signal Sr as an input thereto to produce an impulse response and outputs the impulse response as tap coefficient Tc. Estimated received signal generator 503 receives transmission signal sequences Scs, which are binary numbers representing 0 to P-1 and tap coefficient Tc, as inputs thereto and outputs P estimated received signals Ser.
Maximum likelihood sequence estimation circuit 504 which outputs transmission signal sequences Scs as described above receives P estimated error signals Seerr as inputs thereto and selects a signal sequence corresponding to the one of the estimated error signals Seerr which exhibits the lowest absolute value. Then, maximum likelihood sequence estimation circuit 504 determines one bit of the selected signal sequence as a value for distortion components to be removed from received signal Sr, and outputs detected signal Sd. Automatic equalization of the signal which has been distorted by intersymbol interference is performed by the processing described above.
As described above, since the conventional decision feedback type automatic equalizer produces estimated distortion signal Sed based on detected signal Sd, it cannot remove a distortion precursor component.
Meanwhile, the conventional maximum likelihood sequence estimation type automatic equalizer has another problem in that, since it must perform calculation for, where the number of taps is m, 2.sup.m transmission signal sequences Scs, it involves a very large amount of calculation and requires a very large scale circuit.